Researchers perplexed by tornado clusters’ growing size

An image of a tornado touching down in Oklahoma in May of 1981 from the National Oceanic and Atmospheric Administration’s photo archive. (NOAA/flickr)

Jenna Ladd | December 2, 2016

A recent study in the journal Science reveals that tornado outbreaks are growing in size, and scientists are unsure why.

The study was published just days after 18 tornadoes devastated parts of the Southeast United States Tuesday night into Wednesday morning. The study’s lead author, Michael Tippett of Columbia University, said that 50 years ago tornado clusters, may have involved about 12 tornados, now they average roughly 20. The researchers studied the most extreme outbreaks, which happen about once every five years, and discovered a steady increase in tornado cluster size since the mid-1960’s.

Before the outbreak on Tuesday, 2016 had seen a record low number of tornadoes. In an interview with the Associated Press, Tippett said,“Something’s up. The tornadoes that do occur are occurring in clusters. It’s not any increase in the (total) number of tornadoes.” In contrast with upticks in other kinds of extreme weather, researchers are not seeing a connection between human-induced climate change and larger tornado clusters. Tippett said, “It’s not what we expected. Either it’s not climate change because not everything is, or it is some aspect of climate change we don’t understand yet.”

Other scientists question the validity of Tippett’s study, claiming that increased reporting and the prevalence of urban sprawl may be responsible for the perceived growth of tornado outbreaks. One critic is Howard Bluestein, a meteorology professor at the University of Oklahoma. He said, “It’s a useful exercise, but I would be very, very careful in accepting it.”

Seven people were killed by this week’s tornado cluster, several more were injured.

Iowa Climate Statement 2015: Time for Action

Chris Anderson, Assistant Director of the Climate
Science Program at Iowa State University, spoke during the press conference for the 2015 Iowa Climate Statement at the Iowa capitol in Des Moines on Monday May 11, 2015. (Joe Bolkcom/Twitter)